1. Field of the Invention
The present invention relates to both a method and a system for detecting openings in opaque objects, particularly in converted can ends that are used in beer and beverage containers. More specifically, the present invention provides improvements to photometric end tester systems.
2. Prior Art
Many metallic cans for holding beverages or other products are provided with easy-open can ends, wherein a pull tab attached to a tear strip that is defined by a scoreline in the can end may be pulled to provide an opening in the can end for dispensing the can's contents. Generally, the pull tab is retained to the can end by means of an integral rivet. The manufacture of can ends is a highly complex, high speed operation. Examples of the methods employed in the formation of a can end can be found in U.S. Pat. Nos. 4,465,204 and 4,503,631 both to Kaminski et al, and assigned to the assignee of the instant invention. These patents are incorporated herein by reference as if fully set forth.
Generally, in the manufacture of an easy-open can end, a can end shell is first formed from a metal sheet product, preferably an aluminum sheet product. The can end shell is then conveyed to a conversion press. In the typical operation of a conversion press a can end shell is introduced between an upper tool member and a lower tool member. A press ram advances the upper tool member toward the lower tool member in order to perform any of a variety of tooling operations such as rivet forming, paneling, scoring, embossing, tab securing, and final staking. One of the most difficult and complex operations is the formation of the rivet, a multiple station operation.
It is to be appreciated that the industry is constantly striving to reduce the gauge of material from which can ends are manufactured. For example, most existing ends are dimensioned as 206, 204, or 202 diameter ends. A "206 diameter" end means that the end is 2 and 6/16 inches in diameter and a "202 diameter" end means that the end is 2 and 2/16 inches in diameter. As the can end has become smaller and smaller in diameter, so also has the gauge of the metal used to fabricate the end been reduced. The 206 diameter end was typically manufactured from metal having a thickness of between about 0.0096 and 0.0106 inches, while a 202 diameter end is typically manufactured from metal having a thickness of about 0.0088 inch.
The high speed, complex cold-working of a can end shell to produce a converted easy-open can end must be continually monitored in order to ensure that the converted can end meets specifications and does not demonstrate any "leakers". A leaker is a breach in the integrity of the can end that can be in the form of a microscopic pin hole. In order to ensure that the converted can end is consistent with required specifications and does not contain any leakers, it has become the practice to inspect each converted can end. Because the leakers are typically too small to be visible to the human eye, there has been a need for a system which can rapidly detect minute openings in solid materials without sacrificing either the speed or quality of the inspection. U.S. Pat. No. 3,826,923 which is assigned to the assignee of the instant invention discloses such an inspection system. The contents of this patent are incorporated by reference as fully set forth herein. In the operation of the end inspector, each converted can end is deposited onto a transfer belt. The belt conveys the end into a light-tight area referred to as a test station. The station contains a high intensity strobe light located below the end testing area and a photo-multiplier tube located above the end test area. The converted can end is tested by flashing the strobe light and simultaneously monitoring whether the conditioned output signal of the photo-multiplier tube exceeds a preset threshold. Thus, it can be determined if the end is considered a "leaker" and should be rejected. If the end is judged satisfactory, it continues through the machine and exits to a downstream conveyor system for eventual assembly with a can body to form the finished container.
Because of the high speed production of converted can ends in a conversion press, the desire to use thinner metal and the sophistication of the conversion process, there is a need in the industry that the end testing equipment be able to identify smaller and smaller irregularities or leakers without contributing additional cycle time to the manufacturing process.
it is therefore an object of this invention to provide a reliable method for detecting at high rates of speed defects in converted can ends.
It is another object of this invention to provide an apparatus for effectively practicing the improved method of this invention.
It is another object of this invention to facilitate the enhancement of existing inspection systems through the use of a relatively uncomplicated system for upgrading such existing systems to incorporate the improved method and apparatus of the instant invention.
It is yet another object of this invention to provide a method for increasing the signal to noise ratio of leak identifying radiation transmitted through a defective converted can end.